In Japanese Patent Laid-open No. 1997-235282 (European Patent No. EP 0737682 B1), there has been described a specific kind of 2-alkyl-3-aminothiophene derivatives which has strong control effect against various plant disease and a process for production thereof. As one of processes for producing 2-alkyl-3-aminothiophene derivatives that are useful intermediates of the above compound, there has been known a process for producing it via 2-alkenyl-3-aminothiophene derivatives as intermediates by directly introducing alkyl groups into the 2-position of 3-aminothiophene derivatives. For example, in Japanese Patent Laid-open No. 2000-327678, there has been disclosed a method for synthesizing 2-alkenyl-3-aminothiophene derivatives by reacting 3-aminothiophene derivatives with various ketones and synthesizing 2-alkyl-3-aminothiophene derivatives by the reduction. However, according to the method as described in the document, to introduce alkenyl groups into the 3-aminothiophene derivatives, a formyl group, an acyl group or a carbamate group is needed as a protecting group of an amino group, and there are many rooms of improvement from the viewpoint of economical efficiency.
As a method for introducing alkyl groups into 3-aminothiophene derivatives without using a protecting group, there has been described, in Tetrahedron Letters, 34, 5715-5718 (1993), Journal of Heterocyclic Chemistry, 33, 9-16 (1996) and Tetrahedron, 54, 9055-9066 (1998), that 2-alkyl-3-aminothiophene derivatives are obtained by reacting 3-aminothiophene with various aldehydes in the presence of p-toluenesulfonic acid and selenophenol.
However, in the above documents, there has been no description of the reaction of 3-aminothiophene with a ketone.
Furthermore, in Reference Example 1 of Japanese Patent Laid-open No. 2000-327678, there has been described that when a ketone having reactivity inferior to that of aldehyde is used, 3-aminothiophene is preferentially decomposed due to its instability without progressing the reaction of the ketone with 3-aminothiophene under the conditions described therein, and desired 2-alkenyl-3-aminothiophene derivatives could not be obtained.    Patent Document 1: Japanese Patent Laid-open No. 1997-235282 (European Patent No. EP 0737682 B1)    Patent Document 2: Japanese Patent Laid-open No. 2000-327678    Non-patent Document 1: Tetrahedron Letters, 34, 5715-5718 (1993)    Non-patent Document 2: Journal of Heterocyclic Chemistry, 33, 9-16 (1996)    Non-patent Document 3: Tetrahedron, 54, 9055-9066 (1998)